The present disclosure relates to a single crystal layer formed of gallium nitride (GaN) or a nitride of gallium and other metal, and a method for forming the single crystal layer. The present disclosure also relates to a method for preparing a substrate used in manufacturing electronic or photo-electronic devices including the single crystal layer. The present disclosure pertains to a technical field for forming a nitride based semiconductor material layer on a substrate, and more particularly, to a technical field for preparing a substrate for forming a high quality nitride based semiconductor layer.
Semiconductors based on nitrides of Group III elements or Group V elements already hold important positions in electronic and photo-electronic fields, which will be important more and more. In fact, the nitride based semiconductors may be used in a wide range of fields from laser diodes (LD) to transistors operating at high frequency and high temperature. The nitride based semiconductors may also be used in ultraviolet photo-detectors, surface acoustic wave detectors and light emitting diodes.
Particularly, gallium nitride is widely known for its usefulness in blue light emitting diodes and high frequency and high temperature transistors. However, it is also being extensively researched for use in microelectronic devices. As used herein, gallium nitride includes gallium nitride alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and aluminum indium gallium nitride (AlInGaN).
To grow a gallium nitride layer of low defect density is important in manufacturing gallium nitride microelectronic devices. One of the causes known to generate the defects is a substrate on which gallium nitride is grown. However, it is difficult to prepare a gallium nitride substrate or a substrate for growing gallium nitride without defects. Since the gallium nitride is difficult to melt, typical methods such as Czochralski method where the crystal is grown from a melt cannot be used in producing gallium nitride single crystal for the substrate. Surely, gallium nitride can be molten under ultrahigh pressure, however, this is currently unavailable for commercial use due to the low productivity.
Therefore, in such devices, the most frequently used for growing the gallium nitride layer are sapphire substrate, and next, a silicon carbide (SiC) substrate. However, sapphire is an electric insulator and a poor thermal conductor, and silicon carbide is expensive and has variable quality.
Accordingly, silicon has been proposed as a substitute for sapphire and silicon carbide. Surely, silicon is economically and technologically attractive in comparison to sapphire and silicon. Particularly, silicon is a good thermal conductor and may be easily removed. In addition, silicon is preferred as a substrate material for low cost mass production. This is because there has been developed a silicon based technology system that can be controlled perfectly in industry scale, and a silicon substrate can be manufactured at a much lower cost than the sapphire substrate and the silicon carbide substrate. However, gallium nitride is difficult to grow on the silicon substrate because of the great difference in lattice constant and thermal expansion coefficient between silicon and gallium nitride.
Recently, epitaxial lateral overgrowth (ELO) method is widely used to grow a high quality gallium nitride layer that determines internal quantum efficiency. The ELO method is being used for manufacturing high speed devices such as blue laser diodes, ultraviolet laser diodes, high temperature/high power devices, high electron mobility transistors (HEMT), and hetero-junction bipolar transistors (HBT), by homoepitaxy.
In a typical ELO method, a stripe-shaped silicon dioxide (SiO2) mask is used to reduce stress caused by the lattice mismatch and the thermal expansion coefficient difference between the substrate and the gallium nitride layer. The typical ELO method will be described below with reference to FIG. 1, which shows a cross-sectional view of the substrate for growing gallium nitride according to the typical ELO method.
In the typical ELO method, the gallium nitride layer 2 is grown on the substrate 1 in a furnace. Then, the substrate 1 is taken out of the furnace. After depositing silicon dioxide on the gallium nitride layer in a deposition apparatus, the substrate 1 is taken out of the deposition apparatus. The silicon dioxide layer is patterned using a photolithography technique to form a silicon dioxide mask 3 on the gallium nitride layer 2, and then the substrate 1 is placed again in the furnace so that an ELO gallium nitride layer 4 is grown on the gallium nitride layer 2.
A portion of the ELO gallium nitride layer 4 that is laterally grown over the silicon dioxide mask 3 has relatively high quality compared to a vertically grown portion. This is because it is difficult to propagate defects such as dislocations through the laterally grown portion. Therefore, by forming a device in the portion of the ELO gallium nitride layer 4 that is laterally grown over the silicon dioxide mask 3, excellent properties can be obtained.
However, the ELO method requires the above described complex process such as an additional external process for forming the silicon dioxide mask, increasing process time and process cost. In addition, recently, as a plurality of silicon dioxide masks are used to improve and enlarge the function of the ELO, the number of the processes of forming the silicon dioxide mask and growing the gallium nitride layer is also increased correspondingly. Consequently, this may result in increased process cost, process complexity, time loss and economical loss, and thus in decreased process yield.